The demand for heavy crudes such as those extracted from oil sands has increased significantly in order to replace the declining reserves of conventional crude. These heavy hydrocarbons, however, are typically located in geographical regions far removed from existing refineries. Consequently, the heavy hydrocarbons must be transported via pipelines to the refineries. In order to transport the heavy crudes in pipelines to existing refineries they must meet pipeline and refinery specifications. The solids content/level in the transported hydrocarbons must not exceed set specifications. For example, the pipeline specification for basic sediment and water (BS&W) is a maximum of 5000 wppm. The refinery specification for filterable solids is a maximum of 300 wppm which is more stringent than the pipeline specification.
Mineable oil sands contain bitumen, water and mineral matter. Upgrading or partial upgrading to remove water, mineral matter, and some of the asphaltenes contained in bitumen is required to meet pipeline and refinery specifications cited above prior to transport and further processing. Measurement of solids content in bitumen-froth, bitumen-froth solvent mixtures, bitumen-solvent mixtures and bitumen formed during the upgrading process is an important aspect of process control to meet pipeline and refinery specifications. The composition of an oil sand can vary from region to region, as well as within a region. Continuous monitoring and adjustment of the upgrading process is warranted to ensure the product falls within the specifications.
Upgrading units and partial upgrading units located proximal to the oil sands generally employ a two-step process of extraction and separation prior to pipeline transport.
In the extraction step air and chemicals may be added to a bitumen/water/sand slurry to help separate bitumen from the bulk of the sand, clay and other mineral matter. The bitumen attaches to the air bubbles and rises to the top of the separator to form a bitumen-rich froth containing residual solids and water as impurities while the majority of solids settle to the bottom. Paraffinic or other solvent is added to the bitumen-froth and the mixture is pumped to another separation vessel (froth separation unit or FSU). The addition of paraffinic solvents such as propane, butane, pentane etc. promote the precipitation of asphaltenes in the froth separation unit and helps to remove the residual solids and water impurities that readily settle and resulting in a dry bitumen product that meet specifications cited earlier. When a paraffinic solvent is used in froth separation, the product is referred to as a paraffinic froth-treated bitumen (PFT bitumen). The degree of deasphalting can be controlled by the temperature, type and amount of solvent used in the froth separation unit. A high temperature paraffinic froth-treatment (70-90° C.) improves the performance, for example increases the settling rate of the precipitated solids compared to lower temperature operations.
The partial upgrading process targets removal/precipitation of about 50% of the asphaltenes prior to pipeline transport. A partially upgraded product can be blended with either condensate or synthetic crude oil to meet the pipeline viscosity and density specifications. The total filterable solids in the blended product must be less than 300 wppm to meet refinery specifications. Filterable solids as measured by ASTM-D4807 is a key specification which limits the design and operation envelop of the upgrading unit.
The filterable solids content of a deasphalted bitumen product plays a significant role in the design and operation of the froth separation unit and the upgrading unit as a whole. Conventional methodologies such as cited in ASTM D4807 to analyze the filterable solids require hours to complete. A time lag on the order of four to six hours may be experienced between obtaining a sample and completing a measurement. Thus, should an undesirable measurement be noted, four to six hours of potential off-spec production would have occurred prior to adjustment or shut-down of the upgrading unit.
Filterable solids concentrations can be determined from particle size distribution and particle count measurements. Methods to effectively measure the particle size distribution of the solids in bitumen froth and bitumen-solvent mixtures provide important feedback on the operation of the unit and thereby minimize or eliminate upsets, unplanned unit shut downs, and production of off-spec product. A variety of techniques are available for determining a particle size distribution and particle count measurements. Such techniques include optical, laser diffraction, electrical counting and ultrasonic instrumentation.
The high concentration of solids and the opaque nature of the bitumen-froth and bitumen-solvent mixtures make it difficult, if not impossible to obtain on-line particle size distribution measurements. In addition, fouling due to the high concentration of solids and asphaltene precipitation can severely impact the operability of these instruments.
Wiehe and Kennedy, in their publications entitled The Oil Compatibility Model and Crude Oil Incompatibility (Wiehe et al., Energy & Fuels 2000, 14: 56-59); and in Application of the Oil Compatibility Model to Refinery Streams (Wiehe et al., Energy & Fuels 2000, 14: 60-63) discuss an oil compatibility model in which solubility and precipitation of asphaltenes from oil is determined on a toluene-heptane scale. The model is used to determine if a crude oil mixture experiences dissolution or precipitation of asphaltenes at different solvent ratios. This parameter has been used to determine correct proportions and order of blending crude oils for desired proportions, and can be used in preventing fouling of equipment due to unexpected precipitation of asphaltenes from a crude oil stream.
A relatively dilute concentration of solids is desirable when determining particle size distribution. Dilution of bitumen-froth and bitumen-solvent streams prior to analysis of particle size distribution is desirable, not only to permit accurate analysis but also to prevent fouling of the instrumentation. This is especially important for on-line techniques in which periodic or constant sampling is relied upon to provide feedback to an ongoing upgrading process. However, dilution of a bitumen-containing sample usually leads to a change in asphaltene solubility, and consequently to an inaccurate particle size distribution measurement. Dilution of a sample from a bitumen-containing stream using a typical paraffinic solvent would have the effect of either solubilizing or precipitating asphaltenes, thus leading to a lower or higher solids content in the sample than in the stream. There remains the conundrum that measurement of particle size distribution is best conducted with a diluted sample, but diluting the sample alters asphaltene solubility.
It is, therefore, desirable to provide a method to effectively measure the particle size distribution of the solids in bitumen froth, bitumen froth-solvent mixtures and bitumen-solvent mixtures. Either the particle size distribution and or solids content derived from the same would then assist in providing feedback on the operation of the upgrading unit, thereby minimizing or eliminating upsets, unplanned unit shut downs, or production of off-spec product. Further, such a method could help in optimizing the design of the commercial upgrading processes.
It is desirable to decrease the time-lag between sampling a stream from the upgrading process and obtaining a particle size distribution measurement from the sample.
Further, it is desirable to find a diluent for addition to a bitumen-containing sample that would allow particle size distribution measurements using optical, laser diffraction, electrical counting or ultrasonic techniques without changing the level of deasphalting and fouling.